Not giving up at all. There are those that will never change. It's those I'm speaking of. Sorry, I should have qualified that.
I get your point. Arguing on forums is often frustrating because people rarely step back, look at the discussion and admit or realize that they were wrong.
The discussion is still not in vain, though. Even if the sexist guys won't reconsider their behaviour there will always be a couple of people who read this thread and actually get it. People who otherwise just never would have thought about these issues.
So even if you can never (or rarely) convince those who won't change (or don't want to change) the discussion might still have an influence on the audience.
The audience is part of the discourse even if most people don't realize it. By shaping the discourse we're changing things already. In the best case we're changing the way the audience sees things and "constructs" their reality.